1. Field of the Invention
This invention relates to the field of chemical detection using cantilevered microelectromechanical structures (MEMS) devices and particularly to methods for detecting specific chemical entities using uncoated cantilevers by analysis of selectively excited adsorption-induced surface states upon exposure to photons having discreet energy levels.
2. Prior Art
The microcantilever sensor is a simple sensor concept with extreme high mass sensitivity or surface stress sensitivity. A sensitivity of sub-picograms has been demonstrated for microcantilever sensors and there exists the potential of achieving a mass sensitivity in the range of sub-femtograms. These microcantilevers can be made with a length ranging from one micron to a few hundred microns. The microcantilever resonance frequency changes due to adsorption-induced mass loading. The cantilever can also undergo bending due to adsorption-induced surface stress variation if the adsorption is confined to a single surface. The extreme mass sensitivity of the microcantilever is due primarily to its own extremely small mass. Microcantilever sensors based on mass detection were discussed in U.S. Pat. No. 5,445,008 to Wachter and Thundat. Microcantilever chemical sensors based on variation surface stress is discussed by Thundat and Wachter in U.S. Pat. No. 5,719,324.
The primary disadvantage to using microcantilevers, as well as other forms of mass sensor such as quartz crystal microbalances, surface acoustic wave devices, plate wave resonators, Lamb and Love wave sensors, is the inability to distinguish different chemical entities. As a result, to direct adsorption to a single surface, it has been known to apply coatings of various types to the cantilever. For example, coating the surface of a mass sensor with a thin layer of gold makes the sensor sensitive to mercury adsorption, but not selective because it also becomes sensitive to hydrogen sulfide and other sulfur compounds. Filters and other fixes to attain greater selectivity detract from the primary advantages of microcantilevers, small size and simplicity.
An alternative approach to chemical sensing is the use of arrays of mass sensors wherein each element is coated with a chemical coating selected to produce an unique and characteristic response. This requires many sensor elements and different chemical coatings. This also requires the use of neural networks for correct computation. Finding chemicals and unique combination of chemicals that can act as chemically selective or partially selective coatings is time consuming. Reliably applying the coatings to miniature sensors can be a challenging problem. Moreover, in a micromachined array, it is difficult to make all the individual cantilevers identical. Finally, the sensing based on a chemically modified single cantilever may not be reliable or reproducible due to a number of reasons including contamination. Therefore, chemical sensors based on chemically selective coatings do not offer a clear path to the development of reliable sensors that are miniature, easy to mass produce, and reliable.